The invention relates to multi-channel, multi-loudspeaker surround sound systems, and more particularly to systems having more loudspeakers than independent channels of signal information. More particularly, the invention concerns improving the directional accuracy of reproduced sound images over a wide listening area with minimal unnatural side effects.
A conventional two channel, two speaker stereo system can create sound source images (musical instrument, voice, etc.) along a line between the speakers for listeners located nearly equidistant to the speakers, with only limited psychoacoustic effects outside that line. To create directional effects around the listener or to enlarge the optimum listening area, more speakers are needed. For example, four speakers may be equally spaced around the listening area. Another example is the Dolby Stereo system for motion picture theatres, in which a line of three speakers is placed behind the screen and an array of speakers is disposed to the sides and rear of the audience; each of the speakers receive common signals.
While it would, of course, be desirable to have independent control over each of these typically four speaker signals, more often, particularly for the home consumer, only the two conventional stereo channels are available as independent signals. The large screen (70 mm) Dolby Stereo format does have four independent channels, but the smaller screen (35 mm) format has only two. It is this latter format's soundtrack which serves as the source for present stereo home video movie formats (videocassettes, videodiscs, stereo broadcast television).
In the two channel video soundtrack as well as other two channel surround sound formats, the directional information is encoded using the parameters of relative channel amplitude and/or phase according to the general principle disclosed in Scheiber, U.S. Pat. No. 3,632,886. This encoding may be a matrixing of four source channels into two, or a more general formula of continuous directional specification. In either case four decoded speaker signals cannot be recovered with complete signal independence. Most typically, sound coming predominantly from one speaker will also appear at only a 3 dB lower level in two others, typically adjacent speakers. For listeners near the center of a regular speaker array, this is not necessarily a serious problem, but for listeners outside the central region, or for other speaker arrays, it can lead to a significant loss of directional stability with varying listening locations. The apparent sound source will move strongly towards the closest and apparently loudest speaker and may also become more vaguely located.
For example, in the Dolby Stereo speaker arrangement, center speaker dialogue would appear at a 3 dB lower level in left and right. An off-center listener would hear the dialogue pulled away from center toward his direction, albeit to a lesser extent than would be the case in conventional two speaker stereo. A left speaker's sound effect would appear at a 3 dB lower level in the center speaker and the side-rear array leading to obvious localization difficulties, particularly for listeners near one of the side-rear speakers. A side-rear directional effect would appear at a 3 dB lower level in the left and right speakers, significantly reducing its directional accuracy, particularly for listeners near the front. Similar problems would exist for an image "panned" to left-center, for example, wherein the image appears equally in the left and center speakers and at a 7.7 dB lower level in the right and rear speakers.
Numerous means have been proposed to minimize this sort of problem and to make the four speakers sound more "discrete", concentrating for the most part on images located at the speakers. Of necessity all make some measurement of the predominant direction of sound at each moment and vary the speaker signals responsively to reduce the effect of unwanted leakage signals.
In Scheiber's U.S. Pat. Nos. 3,632,886 and 3,959,590 and in Bauer's U.S. Pat. Nos. 3,798,373; 3,794,781; 3,812,295 and 3,821,471, the gains (levels) of speakers momentarily deemed to be receiving leakage of the predominant signal are reduced in favor of the predominant speaker. Since level modulations of the individual speakers are bothersome, particularly to listeners away from the center of the array, most later devices involved the audibly less drastic measure of effectively varying the decoding matrix parameters to improve the isolation of a momentarily predominant sound at the expense of increased interspeaker leakage of sounds in other directions.
In Ito, U.S. Pat. Nos. 3,825,684 and 3,836,715 and Tsurushima, U.S. Pat. No. 3,786,193, variable mixing of the appropriate phase is introduced between speakers considered to have the predominant signal's leakage, resulting in leakage cancellation and loss of separation for other signals between those speakers.
In Hiramatsu, U.S. Pat. No. 3,829,615, a similar result is obtained by mixing the predominant sound's speaker signal into the two speaker signals with predominant sound leakage to effect cancellation. Again a loss of separation between the leakage speakers results.
In Gravereaux, et al., U.S. Pat. No. 3,943,287, a similar technique is employed with additional gain control amplifiers to maintain constant total power for the predominant signal as its leakage components are cancelled.
In Willcocks, U.S. Pat. No. 3,944,735, the decoding matrix modification is done with sufficient additional complexity to slightly modify and equalize the power levels in the three nonpredominant speakers, if desired. Approximate sensing is done of all directions around a theoretical 360.degree. listening circle and approximate "directional enhancement" (leakage cancellation) matrix modification is performed for all such directions.
In Olson, U.S. Pat. No. 4,018,992, variable gain amplifiers are used to vary two leakage cancellation parameters per speaker to achieve a similar goal. In this case, the goal is perfect leakage cancellation for all encoded directions, specifically in combined amplitude-phase matrices such as SQ (Bauer, U.S. Pat. No. 3,835,255).
While a progession of capability is evident from this prior art review, all approaches are still bound by the fundamental limitation of two signal channels: only one, or at most two orthogonally encoded directions (meaning statistically independent, not at 90.degree. locations in the listening room) can be reproduced simultaneously with complete leakage signal cancellation. (This is ignoring bandsplitting techniques as suggested by Ito U.S. Pat. No. 3,836,715.) This means that as the encoded soundfield progresses past the very simple, leakage cancellation cannot occur for all the multitude of sound source directions present. As individual sounds become less predominant, attempts at leakage reduction should also reduce, as justice cannot be done to the multiplicity of sound directions. This is explicitly or implicitly done on a proportional basis in all the above cited approaches, as the leakage cancellation is applied at a level which depends on the measured degree of directional predominance of (the leakage's) principal component (Gravereaux, et al. U.S. Pat. No. 3,943,287). Even with complex soundfields where many sounds are only briefly, mildly predominant, some varying leakage cancellation action remains. As explained by Bauer, et al., in relation to their circuit approach (Quadraphonic Matrix Perspective" JAES (Journal of the AES), vol. 21, June 1973). "As various signals become present simultaneously, the logic becomes progressively less active under all conditions. Nevertheless, there is instantaneous dominance of individual signals, allowing an adequate logic action to remain even with a "busy" quadraphonic program, since in this latter case human hearing is unable without much effort to follow the action of individual program sources". While judicious choice of response time constants can minimize the adverse audible effect of the resultant soundfield wandering, the net result of attempting to respond to the numerous simultaneous sound directions must be an unstable, jittery soundfield, simply describable as "fidgety".
This was recognized in the design of the Dolby theater decoder (Model CP-50, Cat. No. 150), which uses integrated circuits based on Willcocks, U.S. Pat. No. 3,944,735, with extensive add-ons to modify the operating characteristics. Only sounds which are strongly predominant in the direction of one of the speakers elicit rapid application of leakage cancellation signals. With complex sounds such as orchestral music lacking strongly predominant sound directions, response time is much slower, giving a longer averaging time and greatly reducing "fidgetiness". However, since most stereo music has more energy towards the center than other directions, this averaging will often result in a partial center "directional enhancement" and an undesirable narrowing of the soundstage. Also, nonspeaker-oriented directions such as left-center (partial dominance of left and center signals) are not responded to as rapidly and have their leakage signals cancelled only approximately.
Given the limitations inherent in any leakage cancellation technique, none of the prior art adequately addresses the goal of accurately cancelling the leakage from individual sounds from any encoded direction while not responding in an undesirable manner to less strongly directional soundfields. A system which does achieve this goal, combined with appropriate sensing and control time constants and characteristics, will give the optimum directional acuity to significant directional effects while minimizing aberrant soundfield wandering for more directionally complex soundfields.
It is therefore an object of the present invention to accurately cancel leakage signals in matrix decoded signals derived from encoded directional information for all encoded sound directions taken individually.
It is another object of the present invention to simultaneously cancel such leakage signals for two orthogonally encoded directional sounds occurring nearly simultaneously.
It is another object of the present invention to sense all encoded sound directions with approximately equal or otherwise preset sensitivity relative to other simultaneously occurring sounds.
It is another object of the present invention to achieve the other objects of the invention while not responding to complex sounds which do not have strongly predominant directions nor responding to non-directional sounds.
It is another object of the invention to provide for reduced or eliminated leakage cancellation for one or more directions on a frequency uniform or frequency selective basis to allow for a missing speaker or a speaker of limited frequency range, or to minimize audible side effects.
It is another object of the invention to maintain appropriate total power relationships for all the sound directions for any possible leakage cancellation condition.
It is another object of the invention to achieve the other objects of the invention using circuitry requiring a minimum of adjustment or precision components.
It is still another object of the invention to provide direction indicating logic signals suitable for center balance indication, control of automatic channel balancing, or other uses.